1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly, to a method and an apparatus for driving a plasma display panel.
2. Description of the Background Art
There is a growing interest in a flat panel display device in which the weight and volume of a cathode ray tube can be reduced. This flat panel display device may include a liquid crystal display, a plasma display panel (PDP), a field emission display, electro-luminescence display and the like. It supplies a digital signal or an analog data to a display panel.
The plasma display panel is adapted to display an image by light-emitting phosphors with ultraviolet generated during the discharge of a gas such as a He+Xe or Ne+Xe gas. This PDP can be easily made thin and large, and it can provide greatly improved image quality with the recent development of the relevant technology.
Particularly, a three-electrode AC surface discharge type PDP has advantages of lower driving voltage and longer product lifespan as a dielectric layer on which a wall charge is accumulated in discharging is employed and electrodes are protected from sputtering generated by plasma discharging.
FIG. 1 is a perspective view showing the construction of a cell of a three-electrode AC surface discharge type plasma display panel.
Referring to FIG. 1, a cell of a three-electrode AC surface discharge type plasma display panel includes a scan/sustain electrode 30Y and a common sustain electrode 30Z which are formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18.
The scan/sustain electrode 30Y includes a transparent electrode 12Y and a metal bus electrode 13Y, which has a line width smaller than that of the transparent electrode 12Y and is formed at one edge of the transparent electrode. The common sustain electrode 30Z includes a transparent electrode 12Z and a metal bus electrode 13Z, which has a line width smaller than that of the transparent electrode 12Z and is formed at one edge of the transparent electrode. The transparent electrodes 12Y, 12Z can be formed using a transparent conductive material, e.g., indium-tin-oxide (ITO). The metal bus electrodes 13Y, 13Z are formed of a metal having high conductivity and serve to compensate for electrical properties of the transparent electrodes 12Y, 12Z having high resistance.
An upper dielectric layer 14 and a protection film 16 are laminated on the upper substrate 10 in which the scan/sustain electrode 30Y and the common sustain electrode 30Z are formed. The upper dielectric layer 14 is accumulated with ionized charged particles generated upon discharging. The charged particles accumulated on the dielectric layer 14 are called ‘wall charge’. The protection film 16 serves to protect the upper dielectric layer 14 from sputtering of the charged particles generated upon discharging and to increase emission efficiency of secondary electrons. The protection film 16 is typically formed using magnesium oxide (MgO).
The address electrode 20X is formed on the lower substrate 18 in the direction where it intersects the scan/sustain electrode 30Y and the common sustain electrode 30Z. A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 in which the address electrode 20X is formed. The lower dielectric layer 22 serves to protect the address electrode 20X and increase optical efficiency by reflecting light that proceeds toward the lower substrate 18 upon discharging.
A phosphor layer 26 is formed on the lower dielectric layer 22 and the barrier ribs 24. The barrier ribs 24 are formed in a direction parallel to the address electrode 20X, and it physically divides cells to prevent ultraviolet and a visible ray generated by the discharging from leaking toward cells that are adjacent to one another horizontally. Therefore, optical crosstalk between the cells is prevented and charged particles generated by the discharging are prevented from moving toward cells that are adjacent to one another horizontally, so that electrical crosstalk between the cells is prevented. The phosphor layer 26 is excited by ultraviolet rays generated upon discharging to generate a visible ray of one of red, green and blue. Inert mixed gases such as He+Xe, Ne+Xe and He+Ne+Xe for discharge are inserted into discharge spaces defined between the upper substrates 10 and the barrier ribs 24 and the lower substrates 18 and the barrier ribs 24.
FIG. 2 shows an example of a sub-field in which a frame period is time-divided into eight sub-fields.
In such three-electrode AC surface discharge type PDP, one frame period is driven with it time-divided into several sub-fields having different numbers of emission as shown in FIG. 2 in order to implement the gray scale of a picture. Each of the sub-fields is divided into a reset period for uniformly initializing all cells, an address period for selecting a cell and a sustain period for implementing the gray scale depending on discharge frequency. For example, if it is desired to display a picture using 256 gray scales, a frame period (16.67 ms) corresponding to 1/60 second is time-divided into eight sub-fields SF1 to SF8 as shown in FIG. 2. Furthermore, each of the eight sub-fields includes a reset period, an address period and a sustain period. In the above, the reset period and the address period of each of the sub-fields are the same every sub-field, whereas the sustain period and the discharging frequency of the sustain increase in the ratio of 2n(n=0,1,2,3,4,5,6,7) in each sub-field.
A method of driving a plasma display panel can be largely classified into a selective write (SW) mode and a selective erase (SE) mode depending on a select mode of a cell.
The selective write mode includes initializing all cells in a reset period and then selecting a cell to be turned on (hereinafter, referred to as ‘on-cell’) in an address period. In a sustain period of the selective write mode, sustain discharge is generated in the on-cell.
In this selective write mode, a scan pulse supplied to the scan/sustain electrode 30Y has a relatively wide pulse width. For this reason, in the selective write mode, the address period becomes long. Therefore, this mode has a disadvantage that it is difficult to secure the sustain period sufficiently.
Meanwhile, the plasma display panel is adapted to implement the gray scale of a picture through a combination of sub-fields and thus has contour noise in a motion picture. If the contour noise is generated, the display quality is degraded. For example, if the left half of a screen is displayed as a gray scale value of 128, the right half of the screen is displayed as a gray scale value of 127 and the screen then moves to the left, a peak white, i.e., a white stripe appears at the boundary between the gray scale values 128 and 127. On the contrary, if the left half of the screen is displayed as a gray scale value of 127, the right half of the screen is displayed as a gray scale value of 128 and the displayed screen moves to the right, a black level, i.e., a black stripe appears at the boundary between the gray scale values 128 and 127.
Methods of removing contour noise of a motion picture may include a method of dividing one sub-field and adding 1 or 2 sub-fields, a method of re-arranging the order of sub-fields, a method of adding sub-fields and re-arranging the order of the sub-fields, an error diffusion method and the like.
If sub-fields are added in order to remove motion picture contour noise in the selective write mode, the sustain period shrinks as much as the address period extends. For example, assuming that sub-fields of the selective write mode extend to 10 and a pulse width of a scan pulse is 3 μs in a plasma display panel having a resolution of VGA 640×480, the sustain period shrinks absolutely as follows. An address period occupied by one frame period of 16.67 ms is 3 μs (a pulse width of a scan pulse)×480 lines×10 (the number of sub-fields)=14.4 ms. On the contrary, a sustain period occupied by one frame period is −0.03 ms in which the one frame period of 16.67 ms minus the address period of 14.4 ms, once reset period of approximately 0.3 ms, an erase period of 100 μs×10 (the number of sub-fields) and a vertical synchronization signal (vsync) marginal period of 1 ms.
In order to solve the shortage of the driving time, a method has been proposed in which a plasma display panel is physically divided and respective screen blocks are driven at the same time. However, this method has a problem in that the manufacturing cost increases since driving integrated circuits have to be added.
Meanwhile, the selective erase mode includes initializing all cells in the reset period and selecting a cell to be turned off (hereinafter, referred to as -cell′) in the address period. Further, in the sustain period of the selective erase mode, sustain discharge is generated within the off-cell.
A scan pulse needed for the selective erase mode can be set to be small compared to that of the selective write mode. Accordingly, in the selective erase mode, the address period is smaller than that of the selective write mode. It is thus possible to secure a sustain period relatively widely. For example, assuming that one frame period is time-divided into eight sub-fields and a pulse width of a scan pulse is 1 μs in a plasma display panel of VGA resolution, an address period occupied by the one frame period is relatively small, i.e., 1 μs (a pulse width of a scan pulse)×480 lines×8 (the number of sub-fields)=3.84 ms. A sustain period occupied by the one frame period is approximately 11.03 ms in which the one frame period minus the address period of 3.84 ms, a vertical synchronization signal (vsync) marginal time of 1 ms, a reset period of 100 μ(the reset period)×8 (the number of sub-fields), and the entire surface writing period. As such, in the selective erase mode, the address period shrinks. Accordingly, this mode has an advantage that it can easily secure a sustain period even when the number of sub-fields extends.
However, in the selective erase mode, the entire cells are turned on in the reset period and black brightness rises in the contrast ratio. Therefore, this mode has a disadvantage that a contrast characteristic is degraded.
The applicant of the present application proposed a method and apparatus (hereinafter, referred to as ‘SWSE mode’) for time-dividing one frame period into sub-fields of a selective write mode (hereinafter, referred to as ‘SW sub-field’) and sub-fields of a selective erase mode (hereinafter, referred to as ‘SE sub-field’) under a given condition in order to solve the shortage of a driving time generated in the selective write mode and lowering of a contrast characteristic generated in the selective erase mode (see U.S. Pat. Publication No. US-2002-0033675-A1).
FIG. 3 shows an example that sub-fields of a SWSE mode are arranged.
Meanwhile, the SWSE mode includes time-dividing one frame period into 6 SW sub-fields SF1 to SF6 each of which selects an on-cell in the selective write mode and 6 SE sub-fields SF7 to SF12 each of which selects an off-cell in the selective erase mode, referring to FIG. 3.
The SW sub-fields SF1 to SF6 can represent 64 gray scales through binary coding. The SE sub-fields SF7 to SF12 can represent 7 gray scales through linear coding. A total number of a gray scale that can be represented through a combination of the SW sub-fields SF1 to SF6 and the SE sub-fields SF7 to SF12 is 64×7=448.
Meanwhile, researches have actively been made into a method in which a PDP operates in the PC mode as well as the AV mode so that it can be used both in a television and a monitor of a computer, a bulletin board, a broadcasting board, etc. In this time, the AV mode refers to an operating mode corresponding to TV on which a motion picture is typically displayed. Meanwhile, the PC mode refers to an operating mode corresponding to the monitor on which a still picture is typically displayed.
Optimal conditions required by the AV mode and the PC mode are different from each other. That is, it is required that in the AV mode, pseudo contour noise that easily appears in the motion picture be reduced, whereas in the PC mode, a picture be represented using a large number of gray scales.